Process for initiating explosive and charge therefor



April 22, 1969 R BERGMANN ET AL 3,439,408

PROCESS FOR INITIATING EXPLOSIVE} AND CHARGE THEREFOR Filed June 29. 1967 FIG.! FIG.2 FIG.3

INVENTORS OSUALD R. BERGIANII JOSEPH BUCHIALD GEORGE COWAN ASITORNEY United States Patent 3,439,408 PROCESS FOR INITIATING EXPLOSIVE AND CHARGE THEREFOR Oswald R. Bergmann, Cherry Hill Township, N.J., Joseph Buchwald, Media, Pa., and George R. Cowan, Woodbury, N.J., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed June 29, 1967, Ser. No. 649,887 Int. Cl. B23k 21/00 U.S. Cl. 29--470.1 2 Claims ABSTRACT OF THE DISCLOSURE Improvement in the explosion bonding of metals which comprises initiating the layer of explosive used therefor interiorly throughout its thickness in such a manner that the metal parts to be bonded are driven together under bonding conditions in the area of initiation until the attainment of steady-state collision conditions, with little or no damage to the metals; and an explosive primer particularly useful for effecting such improvement.

Background of the invert-lion The past few years have witnessed the development and commercial acceptance of metallurgically bonded clad products made by explosion bonding. Techniques for explosion bonding are described in U.S. Patents 3,137,937 and 3,264,731, and in copending, co-assigned U.S. Patent application Ser. No. 503,261, now Patent No. 3,397,- 444. Briefly, the procedure involves propelling metal layers together with an explosive so as to cause them to collide progressively at a velocity which is below 120% of the sonic velocity of the metal in the cladding system having the highest sonic velocity. The collision velocity is the velocity with which the line or region of collision travels along the metal layers to be bonded. The metal layers are initially disposed at a standoff from each other at some angle usually less than and preferably substantially parallel, and a layer of detonating explosive is positioned adjacent the outer surface of at least one of the layers and then initiated so as to cause the required progressive collision. When the metal layers are initially substantially parallel, the collision velocity equals the detonation velocity of the explosive. When there is an initial angle between metal layers, the colllision velocity is somewhat lower than the detonation velocity, larger initial angles giving lower collision velocities with a given explosive. The preferred minimum angle to be produced between the metal layers on impact during the bonding process varies from one metal system to another, being about 4 for some, and higher for others. In general, all other things being constant, the impact angle produced decreases with metal density, increases with explosive loading, and increases with initial standoff or angle between layers to a maximum, thereafter decreasing with larger standoffs. Stated differently, impact angle increases with metal layer velocity at constant collision velocity and passes through a maximum with increasing standoff. At a given standoff, the metal layer velocity is increased by increasing the explosive loading, the minimum metal layer velocity usually required being on the order of 150- 400 meters per second.

The explosion bonding process described above produces a continuous metallurgical bonding of the metal layers. It is often difficult, however to achieve strong bonding in the small area of the interface which is directly beneath the location of the initiator for the explosive layer. Because of the special requirements of the bonding system, conventional point initiators, e.g., blasting caps, generally are not used. Such initiators often are inadequate to assure a rapid and reliable initiation especially of the less sensitive low-detonation-velocity cladding explosives. Furthermore, the impulse which a small point charge at the surface of the explosive layer imparts to the metal layer adjacent to the explosive layer is lower in the vicinity of the initiator than in further-removed areas owing to the divergence of the detention front, this efliciency in impulse in the vicinity of the initiator being accentuated by the lower detention velocity and energy output of the cladding explosive in the diverging region. The low impulse tends to cause the metal layers to collide at too small an impact angle and too high a collision velocity for good bonding to occur in the initiator region. Larger initiators located inside the explosive layer afford improved bonding, but nevertheless are found to produce a certain small poorly bonded area due to the fact that the conditions required for bonding i.e., collision velocity, impact angle, and metal layer velocity, fail to be maintained constantly from the moment of initial impact. Measures which have been suggested in the past for avoiding the production of a poorly bonded area beneath the initiation point, have approached the problem by modifying the metal layer adjacent to the explosive. For example, one method, that described in U.S. Patent 3,258,841, uses a metal extension piece on the metal layer on which the explosive is located, the explosive extending over the extension piece as well and initiated at the edge over the extension piece. In this method the extension piece is expended. Another technique, described in U.S. Patent 3,140,539, is to provide a convex projection in the surface of the cladder layer, i.e., the metal layer adjacent to the explosive, and to initiate the explosive at a location in line with the projection. The extension piece method obviously cannot be used when the added expense of the consumed metal cannot be tolerated. The convex projection method generally is troublesome to manage on a production scale, particularly with thick cladder layers. Thus, in the interests of greater economy and more general applicability, it would be highly desirable to have an improved method for reducing the poorly bonded area in the vicinity of the initiator in the explosion metal cladding process which does not require the use of additional metals or additional steps in the preparative stage of the cladding operation.

Summary of the invention This invention provides an improved method for initiating the explosive in explosion metal bonding which results in a considerable reduction in the area of poor bonding usually found in the vicinty of the initiator and which, in its preferred embodiment, provides substantially complete bonding in the interfacial area between metal layers in the vicinity of the initiator with little or no damage or distortion in the parts being bonded. Thus, this invention provides an improvement in. the process f r metal bonding by driving the metal parts to be bonded together progressively with a layer of explosive adjacent at least one of the parts, which improvement comprises embedding an initiating charge, i.e., an explosive primer, within the explosive layer and initiating the explosive layer interiorly throughout its thickness progressively from the surface thereof remote from the metal part to be driven toward the surface thereof adjacent said part,

' the explosive layer being initiated closest to the axis of initiation in the portion of the layer adjacent the metal, thereby driving the parts together under bonding conditions in the area of initiation, i.e., from the region under the primer until steady-state collision conditions are achieved. Preferably, the initiation of the explosive layer adjacent the metal immediately precedes the impulse applied to the metal by the primer along the axis of initiation so that an annular impulse followed by an axial impulse are sequentially applied to the metal. Usually, the impulse along the axis of initiator is a maximum. Also, the explosive loading of the primer is higher than that of the explosive layer, per unit area of the metal part adjacent to the explosive layer and primer.

Preferred primers used for elfecting the improvement of this invention by insertion and actuation in the explosive layer comprise an explosive charge having a smaller cross-sectional area at one end than elsewhere along its longitudinal axis, detonation of the charge in an axial direction toward said small end along the peripheral portion thereof preceding that along the axis.

Reference herein to a portion of an explosive layer or primer being adjacen the metal layer means that portion closest to the metal, it being contemplated that the explosive and primer can either be in direct contact therewith or separated therefrom by buffer or protective material, e.g., of plastic or rubber sheeting of foam. Layer is used herein to refer to a. body of explosive or metal having substantially greater dimensions in two directions than in the third or thickness direction and is intended to include not only planar bodies, e.g., the layers of explosive used in bonding flat sheets and plates, but also curvilinear, e.g., tubular, bodies. Steady-state collision conditions refer to the conditions resulting when dynamic equilibrium between colliding metal parts is established as the detonation progresses through the explosive layer without influence from the primer. T he collision conditions produced in the area of initiation may be substantially the same as the steady-state conditions so that steady-state conditions may prevail during the entire collision process. On the other hand, while the collision conditions produced in the initiation area may differ from the steady-state conditions, the initial impact conditions and those prevailing during the transition to the steady-state are, without interruption, those required for bonding as described above.

The area or place of initiation refers to the location in the explosive layer where initiation of said layer takes place by detonation of the primer embedded therein, and also to the corresponding location in the metal layers being bonded while under the influence of the detonation of the primer. In the present method, the explosive layer is initiated interiorly throughout its thickness. Thus, it is initiated at an inner surface, i.e., the surface of initiation, which conforms to the configuration of the outer surface of the primer embedded therein. Axis of initiation refers to a line perpendicular to the layer of explosive and passing substantially through the center of the cavity bounded by the surface of initiation,

i.e., through the center of the primer used to initiate the layer. Longitudinal axis of the primer refers to the axis thereof which coincides with the axis of initiation when the primer is inserted into the explosive layer and in which direciton the detonation progresses; in most cases, it is the longest axis of the primer, but where the primer is to be used with thin explosive layers or when a flat disk of explosive is positioned at the initiation end thereof, the longitudinal axis need not necessarily be the longest dimension. Peripheral portion of the primer refers to that portion displaced from the longitudinal axis thereof, usually the outer portion thereof exclusive of the end at which it is actuated and the end opposite thereto.

Description of the drawings In the accompanying drawings which illustrate specific embodiments of this invention and wherein like numerals are used to denote like elements:

FIGURES 1, 2, and 3 are cross-sectional views of some various primers of this invention;

FIGURE 4 is a cross-sectional view of a portion of an explosion cladding assembly employing a primer in accordance with this invention; and

FIGURE 5 is a schematic cross-sectional view of a portion of the assembly shown in FIGURE 4 after detonation of the primer.

Detailed description 0 the invention In its broad aspects, the process of this invention is characterized by two features. First, each explosive layer employed is initiated at an inner surface throughout its thickness, rather than by a primer or initiating device at the outer surface of the explosive layer followed by propagation of a detonation through the layer. This allows greater control of the detonation as it begins at the place of initiation, and the primer gives an increase in impulse to counteract the loss from divergence of the detonation front as described above. Second, the initiation of the explosive layer is closest to the axis of initiation in the portion of the explosive layer adjacent the metal. This means that the surface of initiation of the layer, and therefore also the outer surface of the primer which conforms thereto, is closest to the axis of initiation at the portion thereof adjacent the metal, i.e., that the cross-sectional area of the cavity bounded by the surface of initiation and the primer is smaller at the portion adjacent the metal. As a result, the initial action of the primer is concentrated in the center so that the impact angle is large enough and the collision velocity low enough near the axis to give good bonding. This manner of initiation also minimizes damage to the metal yet permits use of primers of sufficient size to assure initition of the explosive layer with near steadystate velocity. Another very important advantage of the second of the above features is that it permits a rapid enough spread of the detonation through the explosive layer to give a smooth transition to steady-state conditions while bonding conditions are maintained.

The use of a special initiating explosive charge or primer in explosion cladding is often advisable to assure a rapid and reliable initiation especially of the less sensitive of the loW-detonation-velocity cladding explosives. For this reason, the primer should usually itself be more sensitive to initiation than the cladding explosives, and, usually at least in the portions of the charge adjacent to the initiating device therefor, e.g., blasting cap, the primer should have a higher detonation velocity than the cladding explosive. At the same time, to ensure a maximum degree of bonding in the interfacial area between metal layers in the vicinity of the place of initiation, the metal layers adjacent thereto should be propelled in a manner such that the conditions required for optimum bonding, i.e., the impact angle and collision velocity, are achieved from substantially the instant of impact and maintained thereafter. These conditions should be met, of course, by a primer which does not cause significant damage to the metal layer adjacent thereto. These requirements are met by primers used in accordance with the present invention.

In processes employing primers having a constant crosssectional area, the pressure drops abruptly from a maximum under the primer to a value similar to the steadystate detonation pressure of the main charge near the periphery of the primer. If the collision angle near the primer axis is quite large and the distance from the axis to its periphery is small, the adjoining low-detonationvelocity cladding explosive may not impart sufficient irnpulse to the adjoining portion of the metal layer to drive it at sufficient velocity to give a large enough steady-state collision angle and the correct collision velocity for good bonding to occur. Increasing the distance from the axis of the primer to its periphery in an attempt to overcome this problem cannot be undertaken at will since gross impact of the metal layers may result, and the risk of damaging the metal layer becomes high with larger primers adjacent to the layer. In accordance with the present invention, the problem is solved without such risk owing to the flexibility which can be achieved in the impulse/ distance profile from the initiation axis. In the present invention, the decreasing load in primer charge explosive away from the initiation axis results in a gradual change in the impulse imparted to the cladder or driver metal from an axial impulse, usually a maximum, to the impulse achieved with the cladding explosive alone. In the intermediate impulse region, a sufliciently high metal layer velocity can be produced to give the required steady-state collision angle and collision velocity. At the same time, the amount of explosive adjacent to the metal layer can be kept to a desired minimum. The gradual impulse change achieved with the present invention also is advantageous in that much less strain is produced in the metal layer than occurs with an abrupt impulse change. An additional advantage offered by the present invention, inherent in the use of a smaller-area of primer adjacent to the metal layer than would normally be required to achieve proper bonding, is that the severity of the pressure pulse reaching the surface of the metal layer at which bonding is to take place is considerably less than that of a pressure pulse produced from a larger-area charge. This also subjects the metal layer to less strain.

The primer or initiating explosive charge used to effect the process of the invention can have any one of a number of geometric configurations. The charge can be tapered from the large-area end either continuously to the opposite end, e.g., in a conical or frustoconical configuration (FIGURE 3); or to an intermediate location on its periphery, e.g., in a funnel configuration. Alternatively, the tapering of the charge can begin at some intermediate location on its periphery and extend to the smallerarea end, e.g., the charge configuration can be that of a cone or truncated cone with its larger base abutting an end, or portion of an end, of a solid cylinder or disk, as in FIGURE 1. Another charge configuration is one which has a T-shape longitudinal section (FIGURE 2). The peripheral configuration of the charge, and therefore the configuration of the inner surface of initiation of the cladding explosive layer, are not critical, and the shape of the cross-section normal to the axis of initiation can be as desired, e.g., a circle, oval, rectangle, triangle, or other polygon, The choice of a specific charge configuration generally Will depend chiefly on how easy the body is to make, how durable it is, etc.

Various embodiments of primers used in the process of this invention will now be illustrated by reference to the drawings. In FIGURE 1, a primer has a frustoconical lower end portion 1a having a cone angle cc and its larger base abutting the end of a solid cylindrical upper portion 1b, the cross-sectional area of the end of the charge to be positioned adjacent the metal to be bonded being smaller than that of the opposite end of the charge. The charge comprises self-supporting detonating explosive 2, e.g., a cast or extruded plastic explosive, surrounded, at its cylindrical portion, by a thin sheath of a different detonating explosivee 3, which also covers the larger-area end surface of the charge in the form of a thin disk 4. Electric blasting cap 5 is affixed to the center of disk 4 on the longitudinal axis of the primer, indicated by the dotted line. This dual-explosive charge is useful in cases in which the explosive used in the body of the primer is insufficiently sensitive to initiation by a blasting cap, or does not have a sufiiciently high detonation velocity to initiate the cladding explosive rapidly and reliably. In such cases, the sheath and disk can be made of an explosive having a higher detonation velocity than explosive 2.

FIGURE 2 shows a primer having a longitudinal section in the shape of a T. In this case the differential in areas is provided by disk 4, which has a larger surface area than the end of cylindrical explosive 2. Substantially the same charge can be made with a disk of the same size as the end of the cylinder, by wrapping or otherwise forming the additional explosive around the disk.

In FIGURE 3, the primer is a truncated cone of a single explosive 2. At the larger base end of the cone an inert block 6, e.g., a wooden block, is inserted in a manner such that a thin peripheral layer and base layer of explosive surround the block. In this embodiment, central initiation of the base of the explosive cone causes the peripheral portion of the cone to detonate ahead of the axial portion.

In FIGURE 4, a primer 1, in this case a single-explosive charge having the same configuration as the charge depicted in FIGURE 1, is shown with its smaller-area end surface resting on the surface of metal layer 7, e.g., a plate, which is to be explosion-bonded to a second metal layer. Explosive charge 1 has a higher detonation velocity and higher loading than the cladding explosive 8 surrounding it. 9 is a frame, e.g., one made of wood, which is used to hold powdered explosive 8. When blasting cap 5 is actuated, the explosive in the primer is initiated, the detonation travelling axially through primer 1 to propel metal layer 7, as well as in other directions to initiate explosive 8 at its interface with the primer. Because the loading of higher-velocity explosive on metal layer 7 decreases from a maximum at the axis of initiation, the longitudinal axis of the primer, metal layer 7 is subjected to a concentrated initial action in the center, causing the metal to deform, as shown in FIGURE 5. The gradual change in impulse to the lower steady-state impulse produced by the charge 8 alone in adjacent regions permits metal layer 7 to be propelled at sufiicient velocity near the axial region to set up a collision angle, ,8 (FIGURE 5), and a collision velocity which are in the range required for bonding at the axis and adjacent regions without causing significant damage to metal layer 7.

The specific dimensions of the charge vary depending on such factors as the properties of the cladding explosive and primer explosive, the mass of the metal layer to be accelerated, the yield strengths of the metals in the system, the steady-state impact angle and velocity desired, the configuration of the charge, etc. To achieve a desired collision geometry in a given system, larger charges will be required with Weaker (lower weight strength) primer explosives, larger driven metal masses, and higher yield strength metals. Larger charges are required to produce larger impact angles. The length of the primer charge, i.e., the distance between its largerand smaller-area ends, in general is substantially equal to the thickness of the cladding explosive layer which it is to initiate. Longer charges can be used, if desired, e.g., disk 4 in the charge shown in FIGURE 2 can extend over the cladding explosive layer. Generally, the area of the end of the charge adjacent the metal layer is as small as possible to give the required acceleration to the metal layer without causing damage. This end of the charge can be a point, as the apex of a cone; but preferably, for ease of use, this end is a surface. As a practical matter, one can first select a charge configuration, fixing the length of the charge from the cladding explosive layer thickness, and select a desirable size for the smaller-area end surface. As a rule, a 0.250-inch-diameter or an area of about 0.05 square inch is adequate, but larger areas can be employed, especially with thicker metal layers. Generally, the longest cross-sectional dimension, i.e., dimension perpendicular to the initiation axis, of the smaller end surface is at least about equal to, and less than about three times, the thickness of the metal layer adjacent thereto, the larger primers being used with higher yield-strength metals.

The size of the larger area of the primer is selected so as to provide, with the adjacent explosive layer, a sufiicient high metal layer velocity beyond the metal area adjacent the smaller-area end of the primer, and a collision velocity in the range required for bonding. In general, the cross-sectional area of the larger portion of the primer is at least about twice, and up to about four times, the area of the smaller portion thereof. The ratios of these areas may be higher, especially with the T configuration.

The variation in cross-sectional area. is governed by the particular charge configuration. In the case of conical' and frustoconical charges, fixing the length and end surface areas of the charge determines the rate of loading change from the periphery to the axis (i.e., determines the cone angle used). In other cases, the large area can be maintained for a certain length of the charge, with a gradual reduction beginning thereafter, as in FIG- URE 1. The cone angle in these cases depends on where the tapering begins. Preferably, the large area is maintained for at least about 50% of the charge length, and the cone angle at is less than about 85. The large-area portion should not extend farther than about 90% of the charge length, and the cone angle zx should be at least 20.

In the case of T-shaped charges or charges in which the base of a cone or truncated cone abuts a portion of a cylinder or disk, the large area is maintained for a certain length of the charge, with an abrupt reduction, followed by a further gradual reduction in the conical cases. In these cases, the large area usually extends for at least about of the charge length.

The primers of this invention can be made of a single explosive composition or two or more explosive compositions. When a single explosive is used, it must meet the requirements specified above, i.e., be more readily initiated than the cladding explosive and at least a portion thereof have a higher detonation velocity than the cladding explosive. Also, it should be provided in a higher explosive loading than the cladding explosive, per unit area of the metal layer. Generally, explosives which detonate at a moderately high velocity, e.g., in the range of about from 3000 to 7000 meters per second, and primer explosive to cladding explosive detonation velocity ratios of 1.5/1 to 3/ 1, are preferred. The explosive can be self-supporting, e.g., a cast or plastic explosive, or a powdered explosive maintained in a weak container, e.g., cardboard. Typical of the explosives which can be used are PETN, TNT, RDX, HMX and other organic nitrates, nitramines and nitro compounds, inorganic azides, and mixtures of the foregoing alone or with other materials as in cast pentolite and 80/20 amatol.

There are some explosive compositions, e.g., cast trinitrotoluene, which are particularly desirable for use in the present primer in that they detonate at a desired moderately high velocity and are capable of propelling the metal layer properly, but which are in themselves difficult to initiate. Such a composition can be used as the major constituent of the primer with a thin layer or sheath of a cap-sensitive higher-velocity detonating explosive surrounding the composition and a thin disk of the more-sensitive explosive at the initiation end of the primer. This embodiment is shown in FIGURES 1 and 2. Usually the thickness of the cap-sensitive layer is about 0.02 to 0.2 inch. Preferably, the ratio of the detonation velocity of the sheath explosive to that of the major constituent of the primer is 1.2/1 to 3/1. When the sheath explosive has a detonation velocity of about 6000 meters per second or higher, the sheath should extend from the larger-area end of the primer to a location intermediate the ends of the primer, extending for more than 50% of the length of the charge periphery, preferably to about 75%, and preferably no more than about 90% of the length. Extension of the layer of cap-sensitive higher-velocity detonating explosive to the end of the charge which will be placed adjacent to the metal layer is not preferred because stronger shock waves will thereby be produced in the metal, with possible damage thereto. When a moderately high detonation velocity explosive is used in the sheath, e.g., one detonating at about 4000-5000 meters per second, the sheath can extend the full length of the primer periphery.

The use of a higher-detonation-velocity explosive layer around the main primer explosive also is beneficial in producing the desired angle upon impact of the metal layers beneath the initiator. Because the detonation in the outer layer of the primer travels ahead of the detonation in the core, the detonation front impacts against the metal layer as a ring or annulus. This results in a slight inward motion of the metal layer toward the center and subsequent impact from the detonation of the central portion of the charge drives the central portion of the metal layer ahead causing a conical deformation and angular impact. The same effect can be achieved with a single-explosive charge by initiating the periphery of the charge ahead of the central portion as shown in FIGURE 3. The particular explosive composition used to form the outer layer is not critical provided it can initiate the lower-detonation-velocity cladding explosive and the core explosive used in the primer. Although cast or powdered explosives can be used, self-supporting sheet explosives are preferred since they can be applied easily by Wrapping around the core. When a dual-explosive system is used in the primer, the core explosive can be a low-velocity cladding explosive. Thus, the detonation velocity of the major constituent of the dual-explosive primer can be equal to the detonation velocity of the cladding explosive, although it preferably exceeds the cladding explosive velocity by 1.5 to 3 times.

The primer has an initiating device, e.g., a blasting cap, usually positioned substantially at the center of the largerarea end in initiating relationship with the sole explosive in a single-explosive charge, or with a peripheral layer of a more readily initiated explosive, if one is required. If it is desired to delay the initiation of the central portion of the primer so that the detonation at the periphery of the charge runs ahead of the detonation of the central portion, a block of inert material, e.g., wood, can be placed within the charge as is shown in FIGURE 3. Alternately, a line wave generator can be used to initiate the periphery of the primer at its larger end.

As has been stated before, the choice of a specific primer configuration depends chiefly on ease of production and durability. On this basis, the design shown in FIGURE 4 is a preferred one. To provide added control on the production of the desired angular impact beneath the charge, and especially if impact at a single point is desired, the primer shown in FIGURE 1 is especially preferred, option ally with the use of an inert insert beneath disk 4 to cause the peripheral layer 3 to be initiated before the axial region of explosive 2. Thus the primer of FIGURE 2 yields maximum bonding and minimum deformation and spalling of the driven metal layer.

The primer is positioned in the layer of cladding explosive with its longitudinal axis normal to the explosive layer and its smaller-area end resting substantially on the surface of the metal layer adjacent the explosive layer, as shown in FIGURE 4. While in some cases the end of the charge will be in contact with the metal layer, a thin layer of a surface-protective material such as plastic film or tape may be used, if desired. The primer can be placed at any point on the surface of the metal layer, e.g., at an edge, corner, or more centrally located point. Several primers also can be placed along one edge of a large plate. In addition, as shown in FIGURE 4, one cladder and one backer layer can be bonded or, alternately, several cladder layers can be bonded with one explosive layer, or, as shown in the examples, several explosive layers can be used simultaneously. An important advantage of the present invention is that, since complete bonding at the initiation site is possible, the place of initiation need not be restricted to an edge as would be the case if a faulty bonding area occurred in the vicinity of the initiator and the area had to be removed.

The collision parameters desired to be achieved upon detonation of the primer and the initiation of the adjacent cladding explosive vary from system to system and are described more fully in the aforementioned patents and patent application, the disclosures of which are incor porated herein by reference. As a rule, a minimum impact angle of about 4 is desired. In the case of silver alloys, such as is described in the Example 1, an impact angle of about 6.5-7.0 is desirable. This impact angle is the steadystate impact angle which is established beyond the axial region of the initiating charge in a typical cladding arrangement. The axial impact angle can be slightly higher. Usually the impact angle varies from about 4 to 25, and preferably to 15. Impact angles and collision velocities can be measured from framing camera sequences.

As previously indicated, the particular primer configuration varies depending on the particular cladding system used. In general for any particular system, a primer having a length about the depth of the explosive layer, a small end with a diameter about 1 to 3 times the cladder thickness and a large portion having a diameter about 2 to 4 times the small end is selected. Steady-state collision and collision under the place of initiation are compared by framing camera sequences, and, if the impact angle at the place of initiation is too large or too small while the collision velocity is in the proper range, the mass or power of the primer explosive can be reduced or increased, respectively. If, on the other hand, the collision velocity is too low, the size of the larger-area portion of the primer can be increased. If there is excessive deformation, primer mass can be proportionately decreased or a less powerful explosive used. By using a higher velocity explosive adjacent the main charge in the peripheral portion of the primer or by peripheral initiation, the angle of impact is sharpened at the first point of impact at the axis of initiation; hence, a poorly bonded area is substantially eliminated.

The following examples serve to illustrate specific embodiments of the present invention. However, they will be understood to be illustrative only and not as limiting the invention in any manner.

Example 1 A three-layered clad is made as follows:

A 12 inch x inch, 0.25-inch-thick 80/20 silver/ copper alloy cladder plate is positioned on each side of a 12 inch x 10 inch, 1.25-inch-thick 80/20 copper/ silver alloy backer plate with the plate surfaces substantially parallel to each other and facing surfaces at a standoff of 0.125 inch. A primer such as that depicted in FIGURE 1 is placed on the outside surface of each cladder plate at the center of one of the 10-inch sides of the rectangular surface. The positions of the primers on the cladder plates are corresponding positions. The configuration of each primer is that of a truncated circular cone having a 0.5-inch-diameter smaller base, a height of 0.5 inch, and a l-inch-diameter larger base abutting the end of a l-inch-diameter, 1.5-inch-long circular cylinder. Explosive 2 is cast trinitrotoluene detonating at a velocity of 5500 meters per second. A 0.05-inch-thick layer of a sheet explosive comprised of PETN in an organic rubber and a thermoplastic terpene hydrocarbon resin binder (US. Patent 2,999,743) and detonating at a velocity of 6900 meters per second is wrapped around the cast TNT for the 1.5-inch-long cylindrical portion of the charge. A 0.05-inch-thick disk of the same sheet explosive is placed on the large-diameter end of the charge, and a No. 6 electric blasting cap afiixed to the center of the disk.

The primer is placed on the plate with the small base of the cone contacting the plate and abutting the plate edge. A wooden frame is placed around the outer surface of each cladder plate, and a granular explosive composition comprised of 16% trinitrotoluene, 74% sodium nitrate, and 10% sodium chloride is packed into the frame so as to form a Z-inch-thick layer detonating at a velocity of 2100- 2400 meters per second. The granular explosive surrounds the primer completely, the frame extending beyond the edge of the cladder plate at the initiator side by 1 inch.

The blasting caps are actuated simultaneously, initiating the primers which, in turn, initiate the granular explosive adjacent thereto as the detonation passes along the periphery of the primers. Initiation of the granular explosive coupled with the impulse from the primers along the axis of initiation causes the cladder plates to collide progressively with the backer. Examination of the clad product reveals complete bonding in the vicinity of the primers, with no evidence of spalling or indentation of the cladder plates. Framing camera measurements made on the same cladder plate and with the same primer show that a steadystate collision angle of 6.5 is set up within 0.25 inch from a point on the axis of the primer, and that a collision velocity of 2100-2400 meters per second is produced from the instant of impact.

Example 2 The procedure described in Example 1 is followed to produce a titanium-steel-titanium triple clad. The 35-A titanium cladder plates are 11.5 x 12 inches and 0.25 inch thick. The C1008 steel backer plate is 13.5 x 14 inches and 1 inch thick. The standoff is 0.25 inch, an dthe detonation velocity 2200 meters per second. The primer described in Example 1 is used on one cladder plate, the smaller base of the cone having a diameter of 0.625 inch, and the diameter of the cylinder being 1.25 inches. The primer used on the other cladder plate is one having the same core and sheath as that described in Example 1, but one whose configuration is entirely cylindrical (l-inch diameter), the sheath extending the full length of the cylinder. After detonation and cladding, the clad product is inspected and found to be completely bonded on the side initiated in accordance with this invention. Under the completely cylindrical initiator, there is a poorly bonded area of 15 square inches.

We claim:

1. In the process for metal bonding by driving the metal parts to be bonded together progressively with a layer of explosive adjacent at least one of the parts, the improvement which comprises embedding an initiating charge within said explosive layer and initiating said explosive layer interiorly throughout its thickness progressively from the surface thereof remote from the metal part to be driven toward the surface thereof adjacent said part,

' said explosive layer being initiated closest to the axis of initiation in the portion of said layer adjacent said part and said axis of initiation being substantially normal to said surface of said explosive layer adjacent said part, thereby driving said parts together under bonding conditions in the area of initiation.

2. A process of claim 1 wherein initiation of said explosive layer at the surface thereof adjacent the metal part to be driven thereby immediately precedes the impulse applied to said part along said axis of initiation.

References Cited UNITED STATES PATENTS 3,090,306 5/1963 Reuther 102-22 3,120,827 2/1964 Abegg et al. 29-421 3,140,539 7/ 1964 Holzman 29-421 3,280,743 10/ 1966 Reuther 102-24 3,281,930 11/1966 Fordham 29-421 3,364,561 1/1968 Barrington 29-497.5 X

VERLIN R. PENDEGRASS, Primary Examiner.

US. Cl. X.R. 

